This invention relates to the problem of compensating, i.e. eliminating or reducing northerly turning error in navigational instruments such as compasses having elements which are responsive to the earth's magnetic field for use in steering fast moving vehicles such as aircraft.
When an aircraft is making a turn through northerly or southerly courses the magnetic element of its magnetic compass is tilted out of the horizontal plane by centrifugal force. The vertical component of the earth's magnetic field then forces the magnetic element downwardly causing it to turn horizontally during the turn away from the north-south magnetic meridian thus resulting in northerly turning error and an erroneous reading of the aircraft's heading during and at the end of the turn.
Many attempts have been made to solve the problem of northerly turning error. Most of these have involved imposing damping on the compass which is heavy enough to make it very slow in responding to the disturbing force of the vertical component thereby reducing but not eliminating the error. The fault is that the compass is slow in settling down from any disturbance, even though small, with the result that it wanders around the magnetic meridian for a considerable time.
Because of this inherent error in magnetic compasses, aircraft have had to have additional expensive equipment to provide precise turning information.
For many years airplanes have used for this purpose the directional gyro, a delicately balanced instrument whose gyroscopic element tends to remain directionally rigid in space. It is not north-seeking, so it must be manually aligned with the magnetic compass during straight and level flight. Due to friction and imbalances, it will slowly drift away from its setting with the magnetic compass, and must be realigned every fifteen minutes or so. The directional gyro provides precise turning data, which the compass cannot, due to the northerly turning error.
As an improvement, the "slaved gyro" was later developed, whereby a remotely located electronic sensor of the earth's magnetic field was arranged to coerce the gyro into constant alignment with north. Thus the resetting of the directional gyro is done continuously and automatically. This system is now in use on substantially all large aircraft, and is one of the essential components of an aircraft autopilot and of many electronic navigation systems.
But the electronic sensor in the slaved gyro system, sometimes called the "magnetic azimuth transmitter", also has northerly turning error, and at least some of this error creeps into the gyro.
This is because, in order accurately to sense the north-south or horizontal component of the earth's magnetic field, this azimuth transmitter, in the form of a magnetometer assembly, has to be maintained horizontally by a pendulum. During a turn through a northerly or southerly heading, the centrifugal force on the pendulum swings the magnetometer coil out of horizontal so that it responds to the vertical component of the earth's magnetic field with a resulting erroneous response to the horizontal component.
If the sensor could be compensated for the effect of the vertical component, the performance of the slaved gyro system would be improved correspondingly. Or, alternately, if the sensor could be disconnected from the gyro while the aircraft was turning, leaving the gyro to operate solely as a directional gyro, and reconnected after the turn, the turn error in the slaved gyro could be completely nullified by exclusion of the effect of the sensor and its northerly turning error.
The magnetic compass and the magnetic azimuth transmitter may both be described as direction indicating components or elements responsive to the earth's magnetic field, and the problem to be solved is how to eliminate or at least reduce or control by compensation, the inherent northerly turning errors of such components or elements.
It is, accordingly, the broad object of this invention to eliminate or at least reduce northerly turning error in direction indicating devices useful for aircraft and other fast moving vehicles, of the type which includes components or elements responsive to the earth's magnetic field.
More specifically, and in one form of the invention, it is an object to eliminate or at least reduce northerly turning error in magnetic compasses useful in aircraft. In another form of the invention, it is an object to eliminate or at least reduce northerly turning error in slaved gyro systems and other systems which may include sensors of the earth's magnetic field.